APOE is the strongest genetic risk factor for Alzheimer's disease and many of its actions in the brain are mediated by specific apoE receptors. Over the past several years, we have defined various signaling roles for apoE and apoE receptors in the brain, and examined how they affect neuronal physiology and animal behavior. In this proposal, we are focusing on one of the most interesting synaptic apoE receptors, ApoER2, and its mechanism of function as a synaptic signaling molecule. Furthermore, we are addressing how the three human apoE isoforms differ in their interactions with ApoER2, using APOE targeted replacement mice. This project is a collaboration between two investigators at two sites, to make use of common interests in ApoER2 signaling and diverse experimental techniques. By conducting both in vitro and in vivo experiments, we will be able to test the in vivo relevance to in vitro findings and design in vitro experiments to define mechanisms for in vivo findings. There are four specific aims; the first three examine the neuronal signaling functions of apoE isoforms via ApoER2; the last one examines how those functions may contribute to pathogenic processes of neurodegeneration. 1. We will determine the in vivo effects of the three human apoE isoforms on neuronal structure and synaptic function; 2. We will define mechanisms of ApoER2 effects on neuronal signaling in vitro; 3. We will define how ApoER2 affects neuronal signaling in vivo; and 4. We will translate these basic findings into two approaches to protecting against synaptic loss in models with Alzheimer's disease pathological changes. These connected and complementary aims will allow us to define important roles of apoE isoforms in normal neuronal function, which may contribute to the associated risk of AD. These experiments also address new functions of apoE receptors in neuronal signaling, defining mechanisms by which these receptors alter synaptic function. They also explore the role of receptor clustering in normal receptor signaling mechanisms. Finally, they will test two important hypotheses about how APOE genotype alters the risk of Alzheimer's disease, translating preliminary findings about JNK activation and ApoER2 receptor clustering into approaches to prevent the synaptic dysfunction associated with neurodegeneration.